In the strictest sense, composites comprise a mixture of two or more solid phases combined in various proportions to provide a final material which exhibits properties which are a combination of the properties of the constituent phases. Composites, such as wood and steel, have been employed for many millennia. Continuous fiber reinforced composite materials are replacing wood and metals in many high performance structures because of their superior strength, lighter weight, and the ability to tailor the properties of these materials to a given application. More recently, the term "composite" has been used principally to define the class of materials in which a matrix material, such as plastics (both thermosetting and thermoplastic), metals, or ceramics, are reinforced by strengthening fibers or whiskers. The term "fiber" is used herein to include both homogenous strands (e.g., wire) and rope-like products of either twisted filaments (yarns) or untwisted filaments (tows). Short fibers dispersed through the matrix material can impart improvement in mechanical properties, but the degree of such improvement is limited. Therefore, maximization of strength properties requires the use of continuous strengthening fibers. Various methods have been employed for the placement of such continuous fibers, including, for example, hand lay-up of prepreg materials, robotic tape lay-up, filament winding, and braiding. Until recently, hand lay-up methods have dominated for the production of high performance composites, e.g., for the aerospace and sports industries. Although highly labor-intensive, manual placement permits precise placement of fibers to achieve optimum mechanical characteristics. Tape lay-up can be automated to some extent, but robotic tape lay-up machines are relatively expensive as are the pre-impregnated ("prepreg") materials they require. Filament winding and tubular braiding, both of which can use raw materials (fibers and resins) and which can be readily automated are being used to allow cost-effective manufacture of composite parts in large quantities.
Of the latter two processes, filament winding offers the advantage of higher fiber volume fraction for maximization of properties. Moreover, since the wound fibers lay relatively flat over each other, filament winding can provide better translation of fiber modulus in the finished product. However, filament winding has several disadvantages from a manufacturing standpoint: The wound filaments must remain on the geodesic path, otherwise they will slip sideways relative to the intended winding direction. Maintaining the fibers on the geodesic path is especially difficult when winding over a tapered, stepped, or undulating mandrel. An additional manufacturing problem is encountered if a wound fiber "preform" is to be converted into a composite by the Resin Transfer Molding ("RTM") method. The wound preform has very low integrity; the fibers are prone to shifting, either during handling or by the flow pressure created when the resin is injected into the preform during molding. The strength, stiffness, thermal properties, and hygral properties of the fiber composite are sensitive to changes in angle, therefore filament winding is not usually feasible for making RTM preforms. Because of these disadvantages, braiding has become a popular alternative for making fiber-reinforced products, especially as a means of fabricating complex geometry tubular shapes (tapers, steps, undulations, etc.) and for making preforms for RTM molding.
A tubular braider 2 contains two sets of fiber carriers 4, 6 which travel in intersecting serpentine paths about a central space 8. Fibers from the two sets of carriers are interlaced by the motion of the carriers to form a biaxial braid as illustrated schematically in FIG. 7. A third system of fibers can be introduced parallel to the braid axis to form a triaxial construction.
Braiding creates a self-stable fabric which conforms to tapered, non-round, or even stepped mandrels. The unique conformability of braiding has enabled the manufacture of complex structural features such as screw threads which are integral to a composite tube or shaft. The braided fabric has excellent integrity so distortion is minimized when the fabric is converted to a composite by methods such as Resin Transfer Molding (RTM). The interlocked fiber structure increases out-of-plane strength and gives rise to excellent impact resistance as compared to wound or laminated composites.
However, these advantages are achieved at the expense of some in-plane stiffness and strength. The decreased stiffness and strength are due in large part to fiber undulation in the braided fabric. The structural efficiency of the braided fabric is also reduced because undulation reduces the effective fiber volume fraction, especially in a triaxial braid. This problem is compounded when high modulus fibers such as graphite are used. The cross-over action in the braiding process causes damage, particularly to high modulus fibers. The abrasive damage increases with the number of braider carriers because of the greater number of cross-overs prior to the braid convergence point; thus limiting the size of a structure which can be braided with a high modulus fiber. Applicants have found that a very high modulus graphite fiber (UHM, Hercules) braids very well on a small braider (16 carriers), but is unusable on a large braider (144 carriers) because fiber damage is so severe.
To minimize abrasive damage during braiding, fibers are often twisted. The twist reduces strength and prevents the individual fibers from flattening out resulting in greater undulation in the braid. The twist also interferes with resin impregnation into the fiber bundle; complete impregnation is necessary to achieve good shear and compression properties.
Thus, braiding also has some disadvantages. First, the fiber undulation in the braid detracts from optimum translation of fiber modulus--thereby lowering the achievable strength and stiffness of the composite. Second, the crossover action in the braiding process leads to greater fiber damage, especially with high modulus fibers. Third, the twist imparted in the fibers to minimize braiding damage tends to reduce strength and prevents the individual fiber bundles from flattening out--the resulting near circular fiber bundles create a thicker ply with greater undulation.
Attempts have been made to create asymmetrically braided products including composite structures of the resin molded filament or fiber reinforced types. Such attempts are exemplified by the following U.S. Patents:
U.S. Pat. No. 2,494,389, issued Jan. 10, 1950, entitled "Braided Product and Method for Producing the Same";
U.S. Pat. No. 2,608,124, issued Aug. 26, 1959, entitled "Braided Product and Method for Producing the Same";
U.S. Pat. No. 4,550,639, issued Nov. 5, 1985, entitled "Shaped Mechanical Compression Packing";
U.S. Pat. No. 4,672,879, issued Jun. 16, 1987, entitled "Shaped Mechanical Compression Packing";
U.S. Pat. No. 4,719,837, issued Jan. 19, 1988, entitled "Complex Shaped Braided Structures";
U.S. Pat. No. 4,729,277, issued Mar. 8, 1988, entitled "Shaped Mechanical Compression Packing";
U.S. Pat. No. 4,754,685, issued Jul. 5, 1988, entitled "Abrasion Resistant Braided Sleeve"; and
U.S. Pat. No. 4,836,080, issued Jun. 6, 1989, entitled "Vibration Abrasive Resistant Fabric Covering".
U.S. Pat. Nos. 2,494,389 and 2,608,124 disclose a method of braiding of a tubular product of varying diameters over the length of the product by braiding about a mandrel. The braiding consists of a plurality of threads, at least some of which are made up of a plurality of independent strands to form a tubular braided fabric and changing the diameter of the braided product by changing the number of strands making up at least some of the threads entering the braiding operation.
U.S. Pat. Nos. 4,550,639; 4,672,879 and 4,729,277 are all directed to a process for producing a product in the form of a mechanical compression packing of generally rectangular cross-sectional shape in which a plurality of axial warp yarns are placed in a predetermined non-symmetrical pattern such that additional corner fills exist adjacent to the outer corners of the packing material vis-a-vis the amount of corner fills adjacent to the inner corners. The non-symmetry results in longer packing life and reduced shaft wear, when the packing is used to prevent leakage about rotating shafts.
U.S. Pat. No. 4,719,837 is directed to the manufacture of complex shaped braided structures using different numbers of groups of axial yarns relative to groups of braiding yarns via an interlacing pattern, wherein each of the braiding yarns extends in diagonal paths completely through the array to the outer periphery of the structure before any of the braiding yarns extend in the path in another diagonal direction from their reversal point.
U.S. Pat. Nos. 4,754,685 and 4,836,080 create asymmetrical tubular products in which the asymmetry is created by varying the diameters of different yarns such as requiring a nylon yarn to be at least 2 mils larger in diameter than a polyester yarn making up the braided sleeve.